Styrene is one of several vinyl aromatic compounds which has considerable commercial utility. Styrene is polymerized into polystyrene which is a clear, readily colored and easily fabricated plastic with many uses. The efficiency of the polymerization process is dependent on the purity of the monomer starting material. Since the processes for producing styrene, and other vinyl aromatic compounds, will contain various reaction products including benzene, toluene, etc., the mixture is distilled to separate these undesirable contaminants. Unfortunately, the temperatures required for distillation, typically 90.degree. C. to about 150.degree. C., leads to the polymerization of the vinyl aromatic compounds. In order to minimize or prevent polymerization of the vinyl aromatic monomer, it is common to add a polymerization inhibitor to the distillation mixture.
The art discloses a variety of compounds which are claimed to inhibit polymerization. These include U.S. Pat. No. 4,050,993 which discloses the use of N,N-nitroso-methylaniline as a polymerization inhibitor. U.S. Pat. No. 3,988,212 which disclose the use of N-nitrosodiphenyl amine in combination with dinitro-o-cresol. U.S. Pat. No. 4,013,580 which discloses the use of N-nitroso aniline derivatives. U.S. Pat. No. 4,341,600 which discloses the use of a mixture of dinitro-p-cresol and N-nitroso-diphenyl amine. U.S. Pat. No. 4,654,451 which discloses alkyl substituted p-nitroso phenol in combination with p-nitroso phenol. U.S. Pat. No. 5,034,156 which discloses N-nitrosophenyl-hydroxylamine plus hydroquinone monomethyl ether. U.S. Pat. No. 5,396,004 which discloses a phenylene-diamine compound plus a hydroxyalkylhydroxyl-amine compound. U.S. Pat. No. 5,254,760 which discloses 1-oxy-2,2,6,6-tetramethylpiperidine plus an aromatic nitro compound. U.S. Pat. No. 4,929,778 which discloses a phenylenediamine compound plus a hindered phenol compound. U.S. Pat. No. 5,312,952 which discloses the use of the reaction product of a C.sub.9 -C.sub.20 alkyl phenol with sulfuric and nitric acid and optionally an aryl or alkyl-substituted phenylenediamine. WO9503263 which discloses 3,5-di-tert-butyl-4-hydroxy-N,N-dimethyl benzyl amine. EP-697386-A2 which discloses 4-acetylamino-2,2,6,6-tetramethyl piperidine N-oxyl in combination with 4-nitroso phenol. JP0701910-A which discloses phosphite compounds, nitrosoamine compounds or phenol compounds. JP05310815-A which discloses the ammonium salt of N-nitrosophenyl hydroxylamine. JP03149205-A which discloses nitrosophenols plus dicyclohexyl-ammonium nitrate. J01226858-A which discloses the use of substituted nitrosobenzene. U.S. Pat. No. 4,967,027 which discloses p-nitroso phenol plus p-t-butyl catechol. J59029624-A which discloses the use of N-nitroso compound, e.g., N-nitroso-diphenylamine and a catechol, e.g., p-t-butylcatechol.
In contrast to these compounds, applicants have found a series of compounds which are particularly effective in inhibiting the polymerization of vinyl aromatic compounds in general and styrene in particular. These compounds are the N-nitroso derivates of unsubstituted or dialkyl substituted phenylenediamine having the following structure: ##STR1## Both the mono and di-nitroso derivates are useful as inhibitors. The compounds can be used by themselves or in combination with the phenylenediamine.